The application of coatings to glass panes is of advantage for a variety of purposes. For example, the coating can block partially or entirely the passage of light through a glass pane or a portion thereof, or can impart a color to the glass pane for decorative, aesthetic or functional purposes. The coating may be conductive to allow it to be used, for example, in conjunction with a defroster or window-heating unit.
Coatings have been applied from coating compositions which have a pasty to flowable consistency, utilizing screening techniques, especially silk-screening, followed by heating of the glass pane provided with the coating to bond the coating to the glass.
When the term "color" to "coloration" is used herein, it will be understood to mean also black or opaque coatings.
A particularly advantageous application of such a coating is the application of a frame-like opaque coating along the edge of an automotive vehicle windshield, especially when the windshield is to be attached in the vehicle body by the direct-bonding technique i.e. without a distinct frame.
The heat treatment which is effected causes a burning of the coating material into the glass and it is generally desirable to carry out the burning of the coating into the glass in conjunction with another heat treatment to which the glass may be subject, e.g. a bending operation or a prestressing process.
In the past, the coating compositions applied to the glass have generally contained a significant quantity of a glass frit which can be replaced by glass-forming components in the composition.
The coating which results after burning thus has a solid glass-like character.
For reasons which are not clear to date, the ultimate strength of the coated composite is greatly reduced as a result of the coating operation. Indeed, the tensile strength or resistance to rupture as well as the compressive strength of the glass pane drops sharply in the region of the coating to be about 1/3 of its original value.
This problem has been recognized in the past and it is known from German Pat. Document No. DE 34 33 408, for example, to operate with certain glass frits and special glass matrixes and to hold the linear coefficient of thermal expansion of the glass matrix in the temperature range of 20.degree.-300.degree. C. below the linear coefficient of thermal expansion of the glass pane. Subscribing to these conditions has been found to be expensive and it has not always been possible to adjust the conditions with the precision required of them, so that the results have not always been satisfactory.